The development of mass spectrometric methodology for the determination of various aspects of protein structure will be continued. A strategy for the determination of disulfide bridges in proteins is proposed that does not involve prior proteolytic digestion of the protein. It is based on the reaction of cyanide ion with -S-S- bonds accompanied by the cleavage of the peptide bond N-terminal to the cysteine so attached. Mass spectrometric molecular weight determination of the resulting fragments then permits the assignment of the disulfide bridges. The structure of thioredoxin h from wheat and that from the photosynthetic bacterium Chloroflexus will be determined, as well as that of the kidney stone formation inhibitor nephrocalcin. A major effort will be the development of methodology for the measurement of the type and frequency of replacement of one amino acid by another one during protein biosynthesis. These aberrant proteins will be present at very low abundance requiring a novel approach, such as tandem mass spectrometry carried out at high sensitivity. The results are expected to effect a better understanding of protein biosynthesis and may have practical significance for biotechnology.